--> Abstract: Application of a Regional Groundwater Flow Model to Provide Boundary Conditions for Local Coalbed Natural Gas Extraction Models in the Powder River Basin, by Alan R. Dutton and Hung-Lung Chen; #90039 (2005)

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Application of a Regional Groundwater Flow Model to Provide Boundary Conditions for Local Coalbed Natural Gas Extraction Models in the Powder River Basin

Alan R. Dutton1 and Hung-Lung Chen2
1 The University of Texas at San Antonio, San Antonio, TX
2 Marathon Oil Company, Houston, TX

Large volumes of groundwater are produced to extract coalbed natural gas (CBNG). Aquifers that make up CBNG reservoirs in the Upper Fort Union Formation of the Powder River Basin are vertically interconnected and laterally extensive. A calibrated MODFLOW-based numerical model of groundwater flow, previously constructed to predict water-level change in Powder River Basin aquifers for an Environmental Impact Statement (EIS), can provide data for constraining non-zero boundary fluxes in local CBNG-production models. The regional model represents the Wasatch and Fort Union Formations; 4 layers represent coal-bearing units of the Upper Fort Union Formation. Simulated water-level changes reflect assigned rates of groundwater withdrawal and hydrogeologic properties. Vertical components of flow around the basin perimeter, especially on the western side, appear greater than in the basin center. In the upper coal-bearing section, the vertical component of flow is directed upward beneath the Powder River and generally downward elsewhere.

Transcribing predicted fluxes from MODFLOW's cell-by-cell water budget to local reservoir models requires reconciling differences in assumed structure and stratigraphy and extrapolating fluxes from ¼-square mile cells into corresponding faces of the local model. An example shows inflow of as much as 10 Stb/day/ft2 (<2•10-4 m/s) across the eastern face of a local model, reflecting nearby recharge areas. Inflow and outflow across western, northern, and southern faces varies with water withdrawal for CBNG production. Application of a regional groundwater model can improve reserve growth and local reservoir management with better estimates of how much water production is needed to reach pressure-reduction targets.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005